1. Field of the Invention
The present invention relates to charge-coupled devices or CCDs. It more specifically aims at a two-phase CCD array device. A preferred application to a CCD image sensor will be described hereafter, it being understood that the present invention may apply to any type of CCD.
2. Discussion of the Related Art
FIGS. 1A to 1C schematically show a portion of a four-phase charge-coupled image sensor. FIG. 1A is a top view, FIG. 1B is a cross-section view along plane B-B of FIG. 1A, and FIG. 1C is a cross-section view along plane C-C of FIG. 1A.
An N-type doped layer 3 is arranged on a P-type doped silicon substrate 1. Substrate 1 and layer 3 form the photoconversion area of the sensor. The upper portion of the photoconversion area is divided into a plurality of lines 5 separated by insulation rows 7, for example, formed of trenches filled with oxide. Columns of insulated electrodes 9, for example, made of polysilicon, equidistant, and perpendicular to lines 5 are arranged above layer 3. A thin oxide layer 11 deposited at the surface of layer 3 insulates electrodes 9 from layer 3. Electrodes 9, properly biased, define in each line 5 a succession of potential wells where electric charges can be stored. In the shown example, a pixel is defined in each line by four successive electrodes G1 to G4. The potential well corresponding to such a pixel is created by application of a high voltage, for example, on the order of 5 V, to electrodes G2 and G3, and of a low voltage, lower than the high voltage, for example, on the order of 0 V, to electrodes G1 and G4.
During an image acquisition period, the sensor is illuminated and electrons resulting from the creation, by absorption of a photon, of an electron-hole pair in the photoconversion area are stored in the potential wells which fill up proportionally to the illumination of the corresponding pixel. The illumination light needs to cross electrodes 9 and insulation layer 11. The thickness of the active region of the sensor, essentially formed by substrate 1 and layer 3, is sufficient to absorb the photons, whatever their wavelengths in the desired spectrum.
After the acquisition period, a transfer period is provided during which the charges stored in the potential wells are transferred in the direction indicated by arrows 13, in parallel for the plurality of columns and in series for the pixels of a same line 5, towards read and/or storage circuits. The charge shifting is ensured by successive modifications of the voltages applied to the electrodes.
FIG. 2 schematically illustrates a simple four-phase mode of transfer of the charges from one well to an adjacent well by switching, between high and low states, of voltages Φ1, Φ2, Φ3, Φ4 applied to electrodes G1, G2, G3, G4 of each pixel.
At a time t0 corresponding to the end of an image acquisition period, charges, shown by the hatched areas of the drawings, are stored in the potential wells formed by application of a high voltage on electrodes G2 and G3 and of a low voltage on electrodes G1 and G4.
At a time t0+T, T being the period of the clock driving the charge transfer, the voltages applied to electrodes G2 and G4 are switched. Thus, the shifting of the potential wells causes the synchronized shifting of the charge packets to the right. To ease the transfer, electrode G4 will be set to the high voltage before electrode G2 is set to the low voltage.
At a time t0+2T, the voltages applied to electrodes G1 to G3 are switched. At a time t0+3T, the voltages applied to electrodes G2 and G4 are switched. Finally, at a time t0+4T, the voltages applied to electrodes G1 and G3 are switched. Thus, at the fourth clock period after time t0, the charges stored in a potential well under a pixel have been shifted towards a potential well under an adjacent pixel of the same line. At the sensor output, the shifted charge packets may be converted into electric voltages by adapted circuits, to form an image signal.
Of course, the transfer period is short as compared with the acquisition period. As an example, the acquisition period is on the order of from 20 to 50 ms and the electrode switching clock frequency may be greater than 2 MHz, which provides a transfer time shorter than 2 ms for a line of 1,000 pixels and a shifting in four phases.
To decrease the transfer period, a shifting in three phases may be implemented.
A disadvantage of the described CCD is that the light needs to cross the polysilicon transfer control electrodes. Part of the photons are thus absorbed in the electrodes, which decreases the sensor sensitivity, especially in the blue range. Indeed, blue photons are absorbed over a short distance while red photons penetrate deeper into the silicon. To overcome this disadvantage, the transfer electrodes may be arranged next to the photoconversion region rather than above it. However, this solution has the disadvantage of increasing the bulk for a given size of the photoconversion region.
Another disadvantage of this type of sensor lies in the fact that the charge storage capacity associated with each pixel is limited by the electrode surface area and by possible carrier recombinations.